1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to a vertically-aligned (VA) LCD having a TFT substrate with specific protrusion or slit pattern for suppressing disclination.
2. Description of the Related Art
Vertically-aligned (VA) LCD is a mode using a negative liquid crystal material and vertical alignment film. When no voltage is applied, liquid crystal molecules are aligned in a vertical direction and black display appears. When a predetermined voltage is applied, the liquid crystal molecules are aligned in a horizontal direction and white display appears. Comparing to the twisted nematic (TN) LCD, the vertically-aligned (VA) LCD provides higher contrast, higher response speed, and excellent viewing angle characteristics for white display and black display. However, the VA LCD still has a critical drawback of a narrow viewing angle. This poses a problem that the application of the VA LCD is limited.
It is known that viewing angle performance of a VA LCD can be improved by setting the orientation of the liquid crystal molecules inside pixels to a plurality of mutually different directions. European Patent Publication Number 0884626-A2 discloses a VA LCD having domain regulating means for regulating the orientation of a liquid crystal in which liquid crystal molecules are aligned obliquely when a voltage is applied so that the orientation will include a plurality of directions within each pixel.
In FIGS. 1a to 1c, as the domain regulating means, pixel electrodes 12 on a TFT (Thin Film Transistor) substrate are slitted, and the common electrode 13 on a CF (color filter) substrate is provided with protrusions 20. As shown in FIG. 1a, in a state in which no voltage is applied, liquid crystal molecules are aligned vertically to the surfaces of the substrates. When an intermediate voltage is applied, as shown in FIG. 1b, electric fields oblique to the surfaces of the substrates are produced near the slits of the electrodes (edges of the electrodes). Moreover, liquid crystal molecules near the protrusions 20 slightly tilt relative to their state while no voltage is applied. The inclined surfaces of the protrusions 20 and the oblique electric fields generated between the substrates determine the directions in which the liquid crystal molecules are tilted. The orientation of the liquid crystal is divided into different directions along a plane defined by each pair of protrusions 20 and the center of each slit thereby improving viewing angle performance. FIG. 2 shows another example of realizing the domain regulating means by providing protrusions 20 alternately on the electrodes 12 and 13, and coated with the vertical alignment films 22. When no voltage is applied, the liquid crystal molecules are aligned vertically to the surfaces of the substrates in principle. On the inclined surfaces of the protrusions, however, the liquid crystal molecules are aligned at a little tilt. When a voltage is applied as shown in FIG. 2, the liquid crystal molecules are tilted in different directions.
FIG. 3 is a diagram showing a plan view of pixel portions of another preferred embodiment disclosed in EP 0884626. As illustrated, the protrusions 20A and 20B are zigzagged alternately on the TFT substrate and the CF substrate. In this case, it is found that a part of display pixels appears darkened. In the region appearing black in the pixel, the change in the orientation of the liquid crystal molecules is found to be very slow upon application of a voltage. This reduces the contrast and the response speed, thereby deteriorating the display quality.
FIG. 4 is a sectional view taken from line A-Axe2x80x2 in FIG. 3. As shown, the direction of the electric field is inclined near the edge of the pixel electrode 12. The liquid crystal molecules near the edge are tilted in the direction perpendicular to the electric field. Thus, the direction in which the liquid crystal molecules are tilted by the inclined electric field near the edge of the pixel electrode 12 is considerably different from the tilting direction due to the protrusions 20A. This results in disclination generating at the border between the domain formed by the edge of the pixel electrode 12 and the domain formed by the protrusions 20A. Therefore, a blackening region is shown in the vicinity of the edge of the pixel electrode.
EP 0884626 further discloses a specific protrusion arrangement pattern for solving the problem described above. As illustrated in FIG. 5, protrusion wings 52 are formed in the vicinity of blackening regions. This protrusion wing 52 is connected to and integrally formed with the protrusion arrangement 20B formed on the CF substrate. FIG. 6 is a sectional view taken from line B-Bxe2x80x2 in FIG. 5. As shown, the direction in which the liquid crystal molecules are tilted by the protrusion wings 52 near the edge of the pixel electrode 12 coincides with the tilting direction due to the protrusions 20A. Therefore, no region appearing dark in the vicinity of the edge of the pixel electrode is observed. However, when the protrusion wings 52 are formed on the CF substrate, during black display, the liquid crystal lying in the gaps between the protrusions are seen black, but additional light leaks out through the liquid crystal near the protrusion wings 52. Thus, it deteriorates the density for black display as well as the contrast.
The present invention therefore seeks to provide a VA LCD that overcomes, or at least reduces the above-mentioned problems of the prior art.
It is a primary object of the present invention to provide a vertically-aligned (VA) LCD having a TFT substrate with specific protrusion or slit pattern which substantially obviates one or more problems due to limitations and disadvantages of the prior art.
To achieve the object described above, the present invention provides a liquid crystal display device comprising: a first substrate and a second substrate processed for vertical alignment; a liquid crystal having a negative dielectric constant anisotropy and being sandwiched between the first and second substrates; an array of first protrusions on the first substrate, each of the first protrusions being bent in zigzag, the first protrusions being arranged in parallel to one another with a predetermined pitch among them; a plurality of pixel electrodes on the second substrate; and an array of second protrusions or slits provided on the pixel electrodes, each extending in a direction, the second protrusions or slits being arranged in parallel to one another. The liquid crystal display device of the present invention is characterized that the angle included between the edge of the pixel electrodes and the extending direction of the second protrusions or slits is kept at least 135 degrees, and the angles included between the extending direction of the first protrusions and the extending direction of the second protrusions or slits as well as the edge of the pixel electrode are both kept at most 45 degrees. This significantly suppresses disclination occurring at the border between the domain formed by the edge of the pixel electrode and the domain formed by the second protrusions or slits.
In the liquid crystal display in accordance with the present invention, it provides a specific design of protrusions or electrode slits to suppress disclination. Specifically, each of the pixel electrodes has triangular indentations formed corresponding to the bends of the first protrusions, and each of the second protrusions or slits is disposed between the apexes of the triangular indentations and corresponding bends of the first protrusions. In a preferred embodiment, the first protrusions are arranged in a manner that the pitch therebetween coincides with an arrangement pitch of the pixel electrodes. Preferably, the zigzagged protrusions are substantially formed inside areas on the first substrate corresponding to the pixel electrodes on the second substrate.
The liquid crystal display in accordance with the present invention is designed to have a specific protrusion or slit pattern which satisfies that the angle included between the edge of the pixel electrodes and the protrusions or slits is kept at least 135 degrees, and the angles included between the first protrusions and the second protrusions or slits as well as the edge of the pixel electrode are both kept at most 45 degrees. Hence, the display defect resulting from disclination can be avoided.